Method of making a unified pressure-sensitive adhesive tape

ABSTRACT

A pressure-sensitive adhesive tape comprising a plurality of superimposed layers, at least one outer layer being a pressure-sensitive adhesive layer, contiguous layer defining an interface therebetween, each of said layers comprising a photopolymerized matrix of polymeric chains; said polymeric chains extending from the matrix of one of said layers through said interface into the matrix of a contiguous layer; said polymeric chains comprising polymerized monomers having migrated from the matrix of each contiguous layer prior to polymerization, whereby said layers cannot be delaminated.

This is a division of application Ser. No. 084,781, filed Aug. 17, 1987,now U.S. Pat. No. 4,818,610, which is a continuation-in-part of U.S.Ser. No. 900,372, filed Aug. 29, 1986, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns photopolymerizable pressure-sensitive adhesivetapes comprising a plurality of contiguous layers which cannot bedelaminated. Each of the layers comprises a photopolymerized matrix ofpolymeric chains, and at least one of the . outer layers isphotopolymerized to a pressure-sensitive adhesive state.

2. Description of the Related Art

The invention concerns photopolymerizable pressure-sensitive adhesivetapes. U.S. Pat. Re. No. 24,906 (Ulrich), reissued on Dec. 20, 1960,discloses pressure-sensitive adhesive tapes, the adhesive layers ofwhich comprise copolymers consisting essentially of monomers of acrylicacid esters of non-tertiary alkyl alcohols having from 1-14 carbonatoms, and at least one monomer copolymerizable therewith.

U.S. Pat. No. 4,181,752 (Martens et al.) discloses a process for makingpressure-sensitive adhesive tape which involves the photopolymerizationof the alkyl esters of acrylic acid and the modifying monomers to formthe acrylate copolymers. It is also disclosed that the intensity andspectral distribution of the irradiation must be controlled in order toattain desirably high peel resistance and cohesive strength. The processdisclosed is preferably carried out in the absence of oxygen and airwhich inhibit the polymerization reaction. Thus, it is normally carriedout in an inert atmosphere such as nitrogen, carbon dioxide, helium,argon, etc. Air can also be excluded by sandwiching the liquidphotopolymerizable mixture between layers of solid sheet material andirradiating through the sheet material. Each layer must be coated andcured before the addition of another layer.

One embodiment of a pressure-sensitive adhesive tape is commonly calleda "transfer tape" in that it typically has a low-adhesion liner fromwhich it is transferred when used. Such a tape can also be linerless asdisclosed in U.S. Pat. Nos. 2,889,038 (Kalleberg) and 4,522,870 (Esmay).One embodiment of the invention, like the tape of U.S. Pat. No.4,223,067 (Levens), has a foam-like appearance and character, eventhough it is not a foam.

The double-coated pressure-sensitive adhesive tape of U.S. Pat. No.2,889,038, (Kalleberg) comprises a flexible support having on oppositefaces chemically different pressure-sensitive adhesive layers which arephysically incompatible, thus enabling the tape to be wound directlyupon itself into a roll for storage and shipment. The tape is made bysuccessively coating and drying solutions of two differentpressure-sensitive adhesives onto opposite faces of a flexible web. Totest for the incompatibility of the two pressure-sensitive adhesives, asolution of one of the pressure-sensitive adhesives is coated onto anundried coating of the other, and the coatings are simultaneously driedat room temperature for 24 hours to evaporate the solvents. Physicalincompatibility is demonstrated by peeling the dried layers apart.

The double-coated pressure-sensitive adhesive tape of the above-citedEsmay patent is similar to that of the Kalleberg patent except that bothadhesive faces can have truly high performance, and the adhesive layersat the two faces of the flexible web do not need to be either chemicallydifferent or physically incompatible. This is achieved when thepressure-sensitive adhesive at each of the faces is a polymer ofpredominantly alkyl acrylate, substantially solvent-free, andcrosslinked. The Esmay patent states: "It is surmised that if theadhesive were not substantially solvent-free, the solvent would allowthe polymer chains to knit across adjacent convolutions during prolongedstorage in roll form, such that perfect separation could no longer beassured. In the present state of the art, it would not be commerciallyfeasible to coat a pressure-sensitive adhesive from solution and obtaina pressure-sensitive adhesive layer which is substantially solvent-free.To keep the amount of solvent to a minimum, the (Esmay) tape ispreferably made using photopolymerization as in U.S. Pat. No. 4,181,752(Martens et al.)" (col. 2, lines 21-32).

The Esmay patent discloses that a "technique for enhancing immediateadhesion to relatively rough or uneven surfaces is to incorporate glassmicrobubbles into the pressure-sensitive adhesive as taught in U.S. Pat.No. 4,223,067 (Levens)" (col. 4, lines 31,35). Because themicrobubble-containing tape of the Levens patent has a foam-likeappearance and character, it is sometimes called a "foam-like" tape eventhough its pressure-sensitive adhesive layer is substantially free ofvoids except for the hollow spaces within the microbubbles. The Levenspatent in turn teaches that where it is desired to adhere the foam-liketape "to a surface to which its pressure-sensitive adhesive layer wouldnot form a strong bond, it may be desirable to apply to one or both ofits faces of its microbubble-filled adhesive layer a layer of unfilledpressure-sensitive adhesive which is especially selected for adhesion tothat surface" (col. 4, lines 9-15). Such microbubble-free surface layerscan also provide substantially increased cohesive strength, especiallyat high temperatures. Multiple microbubble-free surface layers can havedifferent adhesive properties, each selected for good adhesion to acertain surface. Because the application of those added layerssubstantially increase the cost of the foam-like tape, less expensivefoam-backed tapes have dominated the market for uses requiring immediateadhesion to rough or uneven surfaces.

The microbubbles can be glass as in the examples of the Levens patent,or they can be polymeric as described in U.S. Pat. No. 3,615,472(Morehouse et al.) or U.S. Pat. No 4,287,308 (Nakayama et al.).

SUMMARY OF THE INVENTION

A pressure-sensitive adhesive tape comprising a plurality ofsuperimposed layers, at least one outer layer being a pressure-sensitiveadhesive layer, contiguous layers defining an interface therebetween,each of said layers comprising a photopolymerized matrix of polymericchains; said polymeric chains extending from the matrix of one of saidlayers through said interface into the matrix of a contiguous layer;said polymeric chains comprising polymerized monomers having migratedfrom the matrix of each contiguous layer prior to polymerization,whereby said layers cannot be delaminated.

The novel product differs from other tapes of the prior art in that themonomers of the pressure-sensitive adhesive matrix migrate across theinterface prior to polymerization so that after polymerization theinterface comprises a substantial amount of monomers from both sides ofthe interface. This yields layers which cannot be physicallydelaminated.

The present invention embraces a variety of embodiments. One group ofpreferred embodiments of the present invention is that ofpressure-sensitive adhesive tapes which are at least equal inperformance to multi-layer foam-like tapes of the Levens patent, but canbe produced at significantly lower cost. A second group of preferredembodiments is that of cost-effective, double-coated, pressure-sensitiveadhesive tapes. Such tapes may have identical or differing adhesives ateach surface. Such tapes may further comprise one or more non-adhesivelayers selected from a multitude of polymeric matrices, i.e., flexibleor foam-like supports between the adhesive layers, or releasable liners.

As used herein, the term "tape" includes but is not limited to, thoseadhesive strips which are single-coated adhesive layers permanentlyattached to a backing or support, double-coated adhesive strips havingflexible supports with an adhesive layer on both sides thereof, andadhesive strips with no support or backing, such being typically thoughnot necessarily releasably attached to a low-adhesion liner, andcommonly called "transfer tapes".

The present invention also relates to a process for making apressure-sensitive adhesive tape comprising the steps of:

(1) preparing a plurality of coatable compositions, each of saidcoatable compositions comprising at least one photopolymerizablemonomer; at least one of said coatable compositions being curable to apressure-sensitive adhesive state, monomers of each of said coatablecompositions being copolymerizable when blended and subjected tophotopolymerization conditions;

(2) sequentially coating said coatable compositions to provide aplurality of superimposed layers with contiguous layers defining aninterface therebetween, with one composition which is curable to apressure-sensitive adhesive state being coated as a first or last layer;

(3) permitting migration of photopolymerizable monomers through saidinterface between contiguous layers; and

(4) subjecting said superimposed layers to irradiation to simultaneouslyphotopolymerize said monomers in each layer, and to provide polymericchains comprised of copolymers of photopolymerizable monomersoriginating from contiguous layers extending through said interfacetherebetween; thereby to produce a tape having layers which cannot bedelaminated.

All parts, percentages and ratios described herein are by weight unlessotherwise identified.

DETAILED DESCRIPTION OF THE INVENTION

Each of the layers of tapes of the invention comprises aphotopolymerizable matrix comprising polymeric chains. These matricesmay comprise a multitude of polymers; however, all polymers used in suchmatrices must be photopolymerizable, preferably by the ultravioletportion of the spectrum (220-440 nm). At least one layer must bephotopolymerizable to a pressure-sensitive adhesive state.

Such pressure-sensitive adhesive layer of the novel tape has aphotopolymerizable matrix preferably comprising an acrylicpressure-sensitive adhesive.

The acrylic pressure-sensitive adhesives useful in the present inventionare alkyl acrylates, preferably monofunctional unsaturated acrylateesters of non-tertiary alkyl alcohols, the molecules of which have from1 to about 14 carbon atoms. Included within this class of monomers are,for example, isooctyl acrylate, isononyl acrylate, 2-ethyl-hexylacrylate, decyl acrylate, dodecyl acrylate, n-butyl acrylate, and hexylacrylate. Preferred monomers include isooctyl acrylate, isononylacrylate, and butyl acrylate. The alkyl acrylate monomers can be used toform homopolymers for the photopolymerizable polymer or they can becopolymerized with polar copolymerizable monomers. When copolymerizedwith strongly polar copolymerizable monomers, the alkyl acrylate monomergenerally comprises at least about 75% of the photopolymerizablepolymers. When copolymerized with moderately polar copolymerizablemonomers, the alkyl acrylate monomer generally comprises at least about70% of the photopolymerizable polymer.

The polar copolymerizable monomers can be selected from strongly polarcopolymerizable monomers such as acrylic acid, itaconic acid,hydroxyalkyl acrylates, cyanoalkyl acrylates, acrylamides or substitutedacrylamides, or from moderately polar copolymerizable monomers such asN-vinyl pyrrolidone, acrylonitrile, vinyl chloride or diallyl phthalate.The strongly polar copolymerizable monomer preferably comprises up toabout 25%, more preferably up to about 15%. The moderately polarcopolymerizable monomer preferably comprises up to about 30%, morepreferably from 5% to about 30% of the photopolymerizable polymer.

Where superior cohesive strengths are desired, the pressure-sensitiveadhesive matrix of the novel tape should be cross-linked. Preferredcrosslinking agents for an acrylic pressure-sensitive adhesive aremultiacrylates such as 1,6-hexanediol diacrylate as, disclosed in U.S.Pat. No. 4,379,201 (Heilman et al.), incorporated herein by reference.Crosslinking is especially easy to control when photopolymerizing themonomer in admixture with a multiacrylate crosslinking agent. Othertypes of crosslinking agents are also useful, e.g., any of those taughtin U.S. Pat. Nos. 4,330,590 (Vesley), and 4,329,384 (Vesley et al.),both of which are incorporated by reference. Each of the crosslinkingagents is useful in the range of from about 0.01% to about 1% of thetotal weight of the monomers.

Among pressure-sensitive adhesives which are useful for thepressure-sensitive adhesive layer of the novel tape are those whichbecome tacky only at elevated temperatures, e.g., acrylic copolymershaving average carbon-to-carbon chains of less than 4 carbon atoms orthose comprising a polymer wherein methacrylic acid esters aresubstituted for portions of acrylic acid esters.

Tapes of the invention may comprise more than one pressure-sensitiveadhesive layer. In such tapes, the pressure-sensitive adhesive layersmay comprise similar or different adhesives, in like or unlikethicknesses, having similar or different additives.

Where a foam-like pressure-sensitive adhesive tape is desirable, e.g.,inthose applications requiring adhesion to low energy substrates such aspolyethylene and polypropylene, and high solids automotive paintsystems, a monomer blend comprising microbubbles may be used as abacking or core layer. The microbubbles may be glass as taught in theLevens patent, supra, or they may be polymeric. The microbubbles shouldhave an average diameter of 10 to 200 micrometers, and comprise fromabout 5 to about 65 volume percent of the pressure-sensitive adhesionlayer. The thickness of the foam-like layer should be at least sixtimes, preferably at least 20 times that of each microbubble-free layer.The thickness of the layer should exceed three times the averagediameter of the microbubbles and twice the diameter of substantiallyevery microbubble. The thickness of foam-like layers in preferred tapesof the invention range from 0.3 mm to about 4.0 mm in thickness.

When a microbubble-free pressure-sensitive adhesive tape is desired tobe provided on a substantially non-tacky flexible support film, the filmlayer can comprise substantially the same monomers described for thepressure-sensitive adhesive layer, with different ratios of the acrylicacid ester of non-tertiary alcohol and at least one polarcopolymerizable monomer. The preferred range of the polarcopolymerizable monomer in such a layer ranges from 20% to about 60% ofthe total monomer mix. Such layer may also comprise a crosslinking agentand other photopolymerizable ingredients including, but not limited toalkyl vinyl ethers, vinylidene chloride, styrene, and vinyl toluene,only in amounts that do not detract from the desired properties.

Other materials which can be blended with the polymerizable monomermixture include tackifiers, plasticizers, reinforcing agents, dyes,pigments, fibers and fire retardants.

Tapes of the invention may also comprise a woven or nonwoven scrim.Presence of such a scrim will not inhibit migration of the monomers fromone layer through an interface to a contiguous layer of the tape.

The present invention also relates to a process for making thepressure-sensitive adhesive tape of the invention, comprising the stepsof:

(1) preparing a plurality of coatable compositions, each of the coatablecompositions comprising at least one photopolymerizable monomer: atleast one of the coatable compositions being curable to apressure-sensitive adhesive state, monomers of each of said coatablecompositions being copolymerizable when blended and subjected tophotopolymerization conditions;

(2) sequentially coating,, said coatable compositions to provide aplurality of superimposed layers with contiguous layers defining aninterface therebetween, with one composition which is curable to apressure-sensitive adhesive state being coated as a first or last layer;

(3) permitting migration of photopolymerizable monomers through saidinterface between contiguous layers; and

(4) subjecting said superimposed layers to irradiation to simultaneouslyphotopolymerize said monomers in each layer, and to provide polymericchains comprised of copolymers of photopolymerizable monomersoriginating from contiguous layers extending through said interfacetherebetween; thereby to produce a tape having layers which cannot bedelaminated.

A single-coated pressure-sensitive tape of the invention may be made bythe process above applying the first layer to a low-adhesion carrier,such first layer being photopolymerizable to a pressure-sensitiveadhesive state, then applying one or more contiguous layers of a monomerblend which is photopolymerizable to a non-tacky film, andcopolymerizable with the first layer. A double-coated tape may be madeby following these steps with the application of a second layer which isphotopolymerizable to a pressure-sensitive adhesive state. The monomersfor the pressure-sensitive adhesive layers may be identical, or may beselected to provide differing specific adhesive properties at eachsurface of the tape.

A foam-like pressure-sensitive adhesive tape of the invention may bemade by a process of the invention comprising the steps of:

(1) preparing a coatable composition having ultraviolet-transparentmicrobubbles dispersed therein which comprises at least one monomerphotopolymerizable to a pressure-sensitive adhesive state;

(2) preparing one or more coatable compositions which aremicrobubble-free, and comprises at least one photopolymerizable monomer,said monomer being copolymerizable with the monomer in step 1 whenblended and subjected to photopolymerization conditions;

(3) sequentially coating said coatable compositions of step 1, and step2 onto a low-adhesion carrier to form superimposed layers, contiguouslayers defining an interface therebetween;

(4) permitting migration of photopolymerizable monomers through saidinterface between said contiguous layers; and

(5) subjecting said superimposed layers to irradiation to simultaneouslyphotopolymerize the monomers in each layer, and to provide polymericchains of copolymers of polymerizable monomers originating fromcontiguous layers extending through the interface therebetween, therebyto produce a tape having layers which cannot be delaminated. In thisprocess as well as the more general process described above, monomersfrom each contiguous layer have migrated across the interface, so thatafter polymerization, a matrix of polymeric chains extends across theinterface, substantially comprising monomers from each of the contiguouslayers. It is the formation of such polymeric chains that prevents thelayers from being delaminated. Generally, in the preferred foam-likepressure-sensitive adhesive tapes of the invention, the layer containingthe microbubbles is much thicker than the microbubble-free layer. In analternate process of making a foam-like tape of the invention, step (3)of the above-outlined process may involve first applying a thin layer ofa microbubble-free coatable composition onto the low-adhesion carrier;second, applying a thick coating of the coatable composition containingmicrobubbles; and third, applying a thin coating of a microbubble-freecoatable composition over such layer. After simultaneously irradiatingthese coatings, the resulting pressure-sensitive adhesive layer has athick foam-like core and a thin microbubble-free portion at each of itstwo surfaces. In this tape, as in all double-coated tapes of theinvention, compositions comprising different photopolymerizable monomersmay be used in the first and third layers where such would beadvantageous for the application desired.

The coatable compositions used in tapes of the invention, especially thepressure-sensitive compositions are preferably prepared by premixingtogether the photopolymerizable monomers and the polar copolymerizablemonomer, if used, and photoinitiator. This premix is then partiallypolymerized to a viscosity in the range of from about 500 cps to about50,000 cps to achieve a coatable syrup. Alternatively, the monomers canbe mixed with a thixotropic agent such as fumed silica to achieve acoatable syrup composition.

Photopolymerization is preferably carried out in an inert atmosphere,such as nitrogen. An inert atmosphere can be achieved by temporarilycovering the photopolymerizable coating with a plastic film which istransparent to ultraviolet radiation, and irradiating through the filmin air. If the photopolymerizable coating is not covered duringphotopolymerization, the permissible oxygen content of the inertatmosphere can be increased by mixing the coating with a combustible tincompound as taught in U.S. Pat. No. 4,303,485 (Levens), which alsoteaches such technique for making thick coatings in air.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, the single FIGURE schematically illustrates themanufacture of a preferred pressure-sensitive adhesive tape of theinvention.

As shown in the drawing, onto ultraviolet-transparent transparent,low-adhesion carriers 10 and 12 are respectively knife-coated coatings14 and 16, each comprising a syrup consisting of a monomer blend whichis photopolymerizable to a pressure-sensitive adhesive state. Over thephotopolymerizable coating 14 is knife-coated a very thin coating 18 ofmonomer which is copolymerizable with monomer of the coating 14 and canact as a barrier to migration of monomer into the coating 14 from thenext coating 22 which is thickly applied at the nip of a pair of rollers20. The coating 22 is a mixture of ultraviolet-transparent microbubblesdispersed in monomer which is both photopolymerizable to apressure-sensitive adhesive state and copolymerizable with monomers ofeach of the coatings 14, 16 and 18. Emerging from the rollers 20 are thetwo low-adhesion carriers 10 and 12, between which are the four coatings14, 16, 18 and 22 which simultaneously subjected to ultravioletradiation from a bank of lamps 24, thus photopolymerizing the monomersto provide a layer of pressure-sensitive adhesive which comprises amatrix of polymeric chains that extends across the interfaces between athick microbubble-filled core and the two microbubble-free surfaceportions, resulting from the polymerization of the coatings 14 and 16.

The carrier 10, instead of being low-adhesion, can have anadhesion-promoting treatment, if necessary, in order to create apermanent bond between the pressure-sensitive adhesive layer and thecarrier. A permanently bonded carrier can be selected to provide a tapeaffording good abrasion resistance and/or corrosion resistance and/orenvironmental protection. A permanently bonded carrier can be a hot-meltadhesive by which the pressure-sensitive adhesive layer can be bonded toa substrate such as gasketing rubber. For any of these uses, the tapemay or may not include the thick coating 22.

T-PEEL

T-Peel is measured as in ASTM D-1876-72 except that the test tapes were0.5-inch (1.27 cm) in width and were tested after being adhered toaluminum foil backings for two hours. Results are reported in Newtonsper decimeter (N/dm). Only cohesive failures are reported.

T-Peel provides a quantitative value of cohesive strength and is lesssensitive to differences in the adhesion of the pressure-sensitiveadhesive to the test surface.

180° PEEL ADHESION

The adhesive layer to be tested is transferred to 0.05-mm thick,chemically primed, biaxially oriented poly(ethylene terephthalate) filmwhich then is slit to a width of 1/2 inch (1.27 cm). The resulting tapeis self-adhered to plate glass under the weight of a 2.04-kghard-rubber-covered steel roller, 2 passes in each direction. Afterdwelling at 23° C. for about 5 minutes, "180° Peel Adhesion" is measuredby moving the free end of the tape away from the glass parallel to thesurface of the glass at a rate of about 0.5 cm per second (using atensile tester).

90° PEEL ADHESION

The adhesive layer to be tested is transferred to 0.05-mm thick,chemically primed, biaxially oriented poly(ethylene terephthalate) filmwhich then is slit to a width of 1/2 inch (1.27 cm). The resulting tapeis self-adhered to a smooth stainless steel plate under the weight of a2.04-kg hard-rubber-covered steel roller, 2 passes in each direction.After exposure to the indicated conditions, "90° Peel Adhesion" ismeasured by moving the free end of the tape away from the steel plate at90° and at a rate of about 0.5 cm per second (using a tensile tester).

HOLDING POWER

This test employs two 25.4-mm wide stainless steel straps as follows:Type 304-2BA, 0.38 mm in thickness, surface roughness 0.05 micrometerarithmetic average deviation from the main line. The strips are washedwith heptane (also with MEK if heavy oils are present). A strip of25.4-mm wide double-coated pressure-sensitive adhesive tape, carried ona low-adhesion liner, is adhered to one end of one of the straps andtrimmed to a length of 25.4-mm. The liner is then removed, and the otherstrap adhered to the exposed adhesive surface. The specimen is placed ina horizontal position and rolled down with a 6.8-kg (15-lb.) roller toensure intimate contact between the surfaces. After dwelling at roomtemperature for 24 hours, the assembly is hung in a 121° C. oven fromone strap with a weight attached to the other strap, and the time atwhich the weight falls is recorded. The time of failure is indicative ofthe "Holding Power" of the double-coated pressure-sensitive adhesivetape. If no failure occurs, the test is discontinued after 24 hours.

STATIC SHEAR TEST

An assembly is prepared as in the test for Holding Power except thatinstead of using a roller, a 1-kg weight rests on the assembly for 15minutes at room temperature. Then the panel with the adhered tape isplaced in an air-circulating oven which has been preheated to theindicated temperature, and after 15 minutes, a weight is hung from thefree end of the tape, with the top strap vertical. The time at which theweight falls is the "Static Shear Value". If no failure, the test isdiscontinued at 10,000 minutes (in the 70° C. test) or sometimes at 1440minutes (in the 121° C. test). Only cohesive failures are reported.

DELAMINATION TEST

A specimen of the tape is immersed in a bath of ethyl acetate atordinary room temperature, then visually examined periodically. Anyvisual evidence of delamination is reported as a failure. The test isdiscontinued if there has been no failure after 24 hours.

In the following examples, parts are given by weight. The glassmicrobubbles used in the examples had a density of 0.15 g/cm³ and were20-150 micrometers in diameter (average 55 micrometers).

EXAMPLE 1

A syrup was prepared from 87.5 parts of isooctyl acrylate and 12.5 partsof acrylic acid with 0.04 phr "Irgacure" 651, infra, which had beenpartially polymerized by ultraviolet radiation to a viscosity of 3600cps (Brookfield), an inherent viscosity of the polymer being 2.84 and adegree of polymerization of 8%. After adding 0.1 phr (phr--parts perhundred syrup) of 2,2-dimethoxy-2-phenyl acetophenone photoinitiator("Irgacure" 651), 0.056 phr of hexanediol diacrylate crosslinking agent,and 8 phr of glass microbubbles, the mixture was thoroughly mixed with apropeller mixer at 500 rpm for ten minutes and then carefully degassedin a desiccator using a vacuum pump.

A microbubble-free monomer blend of 90 parts isooctyl acrylate and 10parts acrylic acid (and 0.04 phr "Irgacure" 651) was partiallypolymerized by ultraviolet radiation to a degree of polymerization ofabout 6-10%, followed by the addition of the same photoinitiator and acrosslinking agent, mixing, and degassing. The crosslinking agent usedin the microbubble free monomer blend was Photoactive Triazine B of U.S.Pat. No. 4,391,687 (Vesley), and its amount was 0.15 phr.

Pressure-sensitive adhesive tapes were prepared as illustrated in thedrawing except omitting the coating 18. Used for the low-adhesioncarriers 10 and 12 were a pair of transparent, biaxially orientedpoly(ethylene terephthalate) films, the facing surfaces of which hadlow-adhesion silicone coatings. The thicknesses of the coatings were:

coatings 14 and 16 (microbubble-free blend: 0.05 mm

coatings 22 (microbubble-filled mixture): 1.0 mm

The composite emerging from the roller 20 was irradiated with anexposure of 410 mJ/cm² (Dynachem Radiometer Model 500) from a bank oflamps, 90% of the emissions of which were between 300 and 400 nm with amaximum at 351 nm. The composite was cooled by blowing air against bothfilms during the irradiation to keep the temperature of the films below85° C. to avoid wrinkling of the films.

COMPARATIVE EXAMPLE 1

A double-coated, foam-like pressure-sensitive adhesive tape was madelike that of Example 1 except as follows:

(1) the microbubble-filled mixture was coated and polymerized by itself;

(2) the microbubble-free monomer blend was coated into a low-adhesioncarrier and photopolymerized to provide a transfer tape; and

(3) a length of the transfer tape was hot-laminated onto themicrobubble-filled layer.

The hot lamination was carried out between two rollers, one steelinternally heated to 150° C. and the other covered with silicone rubber.The rollers were spaced 0.5 mm apart and were driven at 30 cm/min. Thesilicone rubber covering had been selected to apply maximum pressurewith substantially no microbubble breakage.

The crosslinked double-coated pressure-sensitive adhesive tapes ofExample 1 and Comparative Example 1 were tested as reported in Table I.

                  TABLE I                                                         ______________________________________                                        Examples:           1             Comp. 1                                     ______________________________________                                        90° Peel Adhesion (N/dm)                                               RT for 20 min.      103           44                                          RT for 72 hr.       226           42                                          38° C. 100% RH for 72 hr.                                                                  174           57                                          Holding Power (minutes)                                                       l-kg weight         1440          57                                          Static Shear Test (minutes)                                                   500 g weight/70° C.                                                                        3000          154                                         250 g weight/121° C.                                                                       1440          <1440                                       Delamination Test (minutes)                                                                       Passed        30                                          ______________________________________                                    

EXAMPLE 2

A double-coated pressure-sensitive adhesive tape was made as in Example1 except as follows. There was only one microbubble-free coating, and itwas partially polymerized 70/30 isooctyl acrylate/N-vinyl-2-pyrrolidone.A 0.025 mm coating (coating 18 of the Drawing) of partially polymerizedisooctyl acrylate was interposed between that coating and the core.

EXAMPLE 3

A double-coated pressure-sensitive adhesive tape was made as in Example2 except omitting the coating 18 of partially polymerized isooctylacrylate. Test results for tapes of Examples 2 and 3 are reported inTable II.

                  TABLE II                                                        ______________________________________                                        Examples:             2      3                                                ______________________________________                                        90° Peel Adhesion (N/dm)                                               RT for 20 min.        43     22                                               RT for 24 hours       153    91                                               RT for 72 hours       131    66                                               ______________________________________                                    

EXAMPLE 4 AND COMPARATIVE EXAMPLE 4

Double-coated, pressure-sensitive adhesive tapes were made as in Example1 and Comparative Example 1 except as follows: Each microbubble-filledmixture was frothed as in the "Typically Tape-making Procedure" of U.S.Pat. No. 4,415,615, and there was only one coating of themicrobubble-free monomer blend. Test results are reported in Table III.

                  TABLE III                                                       ______________________________________                                        Examples:          4              Comp. 4                                     ______________________________________                                        90° Peel Adhesion (N/dm)                                               RT for 20 min.     83             82                                          RT for 72 hr.      136            146                                         Static Shear Test (minutes)                                                   500 g weight/70° C.                                                                       >10,000        4747                                        250 g weight/121° C.                                                                      6,000          4200                                        Delamination Test (minutes)                                                                      Passed         30                                          ______________________________________                                    

EXAMPLES 5-7

Three pressure-sensitive adhesive tapes (transfer tapes) were preparedfrom two unfilled, partially polymerized monomer blends using thefollowing monomers:

    ______________________________________                                        isooctyl acrylate        IOA                                                  butyl acrylate           BA                                                   acrylic acid             AA                                                   N--vinyl-2-pyrrolidone   NVP                                                  ______________________________________                                    

Each of the monomer blends was partially polymerized in the same way aswas the microbubble-free blend of Example 1 except that the crosslinkingagent of the IOA/AA/BA coating of Example 5 was 0.1 phr hexanedioldiacrylate. As in Example 1, each of the coatings was 0.05 mm inthickness. In each case, the two blends were coated sequentially ontobiaxially oriented poly(ethylene terephthalate) film and irradiated asin Example 1 except in a nitrogen atmosphere and at an exposure of 220mj/cm². Upon thus being photopolymerized, polymeric chains were formedwhich extended across the interface between the two coatings of each ofthe tapes.

The tapes of Examples 5-7 were tested for 180° Peel Adhesion withresults as indicated in Table IV.

                  TABLE IV                                                        ______________________________________                                                                   180° Peel Adhesion                          Example Monomers for each coating                                                                        (N/dm)                                             ______________________________________                                        5       98/2 IOA/AA        16                                                         70/15/15 IOA/AA/BA 24                                                 6       70/30 IOA/NVP      50                                                         90/10 IOA/AA       37                                                 7       100 IOA             8                                                         80/20 IOA/AA       24                                                 ______________________________________                                    

Each of the tapes of Examples 5-7 was subjected to the DelaminationTest, and none of them failed.

Each of these tapes can be applied to a backing which provides goodabrasion resistance, corrosion resistance, or environmental protection.In doing so, the face of the pressure-sensitive adhesive layer of thetape that better adheres to that backing would be applied to thebacking. To reduce production costs, the pressure-sensitive adhesivelayer of each of those tapes could be formed directly upon the backinginstead of using the poly(ethylene terephthalate) film.

EXAMPLE 8 AND COMPARATIVE EXAMPLE 8

Two pressure-sensitive adhesive tapes were made as in Example 1 andComparative Example 1, except as follows: Both monomer mixtures weremicrobubble-free, 0.06 phr of a multifunctional acrylate crosslinkingagent, ethoxylated trimethylolpropane diacrylate was used, along with37.5 phr Chemlink 2000™, a long chain activated diol oligomer.

The tapes of Example 8 and Comparative Example 8 were tested fordelamination. The tape of the invention, Example 8, passed. The tape ofthe prior art, Comparative Example 8, failed at 10 minutes.

What is claimed is:
 1. A process for making a pressure-sensitiveadhesive tape comprising a plurality of superimposed layers, at leastone outer layer being a pressure-sensitive adhesive layer, contiguouslayers defining an interface therebetween, each of said layerscomprising a photopolymerized matrix of polymeric chains; said polymericchains extending from the matrix of one of said layers through saidinterface into the matrix of a contiguous layer; said polymeric chainscomprising polymerized monomers having migrated from the matrix of eachcontiguous layer prior to polymerization, said process comprising thesteps of:(1) preparing a plurality of coatable compositions, each ofsaid coatable compositions comprising at least one photopolymerizablemonomer; at least one of said coatable compositions being curable to apressure-sensitive adhesive state, monomers of each of said coatablecompositions being copolymerizable when blended and subjected tophotopolymerization conditions; (2) sequentially coating said coatablecompositions to provide a plurality of superimposed layers withcontiguous layers defining an interface therebetween, with onecomposition which is curable to a pressure-sensitive adhesive statebeing coated as a first or last layer; (3) permitting migration ofphotopolymerizable monomers through said interface between contiguouslayers, and (4) subjecting said superimposed layers to irradiation tosimultaneously photopolymerize said monomers in each layer, and toprovide polymeric chains comprised of copolymers of photopolymerizablemonomers originating from contiguous layers extending through saidinterface therebetweenthereby to produce a tape having layers whichcannot be delaminated.